Why did God create angels?

St. Gregory the Theologian says in his Oration on Nativity that God created angels because of His goodness that He may pour on them from His goodness and that they may enjoy thinking of Him and serving Him.⁶⁵⁸

The Holy Bible explains the relationship of good and evil angels to the creation of humanity. Good angels praised God and rejoiced, while the evil angels envied them, especially Satan.